1. Field of the Invention
The invention relates to a fuel cell battery system and, particularly, to a fuel cell battery system that operates a fuel cell battery while a channel of a fuel off-gas is closed.
2. Description of the Related Art
A fuel cell battery has a fuel cell stack in which a plurality of cells is stacked. Each cell is formed by, for example, stacking a membrane-electrode assembly (MEA) and separators. The membrane-electrode assembly has an electrolyte membrane made of an ion exchange resin, an anode provided on one of two surfaces of the electrolyte membrane, and a cathode provided on the other surface of the electrolyte membrane. Besides, each of the anode and the cathode has a catalyst layer that is disposed in contact with the electrolyte membrane. When each electrode is supplied with a reactant gas, electrochemical reactions occur between the electrodes, and generate electromotive force. Specifically, the reactions occur upon hydrogen (fuel gas) contacting the anode and oxygen (oxidant gas) contacting the cathode.
In general, the cathode is supplied with air taken in from the outside by a compressor. On the other hand, the anode is supplied with hydrogen stored in a high-pressure hydrogen tank. A method of supplying hydrogen to the anode is a dead-end method (e.g., see Published Japanese Translation of PCT application No. 2004-536436 (JP-A-2004-536436)). In this method, the system is operated while a hydrogen channel is closed, so that the anode is supplied with an amount of hydrogen that corresponds to the amount of hydrogen consumed.
In the case of the dead-end type fuel cell battery system, the amount of impurities dwelling in the channel of the fuel gas increases as time elapses. For example, nitrogen contained in the air supplied to the cathode permeates through the electrolyte membrane, and is accumulated in the anode side. Since the anode-side pressure is adjusted so as to be equal to a predetermined value, an increase in the amount of nitrogen relatively reduces the partial pressure of hydrogen, resulting in a reduced voltage of the fuel cell battery. Therefore, in Published Japanese Translation of PCT application No. 2004-536436 (JP-A-2004-536436), a purge valve that discharges impurities from the channel of, hydrogen is provided, and the purge valve is opened so as to recover the voltage.
Generally, the fuel cell stack is provided with a supply manifold that distributes hydrogen to individual cells, and a discharge manifold that collectively discharges fuel off-gas from the individual cells. Besides, the cells have pressure losses that vary due to variations caused in manufacture. In a cell with high pressure loss, hydrogen is less easily introduced into the cell from the supply manifold than in a cell with low pressure loss. On the other hand, the fuel off-gas discharged from the cells to the discharge manifold is likely to be sucked into cells whose pressure loss is high. Therefore, impurities likely to deposit in cells whose pressure loss is high, and the voltage declines starting with such cells. This gives rise to a problem of the amount of electric power generation varying among cells.
In the fuel cell battery system described in Published Japanese Translation of PCT application No. 2004-536436 (JP-A-2004-536436), the voltage of the cells disposed at a gas outlet-side terminal end of the fuel cell stack is measured. By comparing the thus-measured value with a threshold value, the timing of opening the purge valve is determined. However, Published Japanese Translation of PCT application No. 2004-536436 (JP-A-2004-536436) does not pay attention to differences in the amount of deposit of impurities resulting from varying pressure losses of the cells, and therefore may fail to resolve the variations in the amount of electric power generation among the cells.